Grain carts are typically used to transfer grain or other crop material from a combine harvester to a grain truck or bin. The grain cart typically comprises a storage hopper for crop material, an auger assembly for emptying crop material from the storage hopper and a set of ground wheels so that the grain cart can be towed. A tractor is commonly used to tow the grain cart and the auger assembly of the grain cart can be driven off of the power take off (PTO) of the tractor (although hydraulics, etc. could also be used to power the auger assembly).
In use, the grain cart is towed by the tractor to a combine harvester where the crop material is unloaded from the combine harvester into the grain cart. With the crop material loaded into the storage hopper of the grain cart, the tractor can tow the grain cart to a truck, trailer, storage bin, etc. and use the auger assembly to unload the crop material from the grain cart into the truck, trailer, storage bin, etc.
If an operator wants to unload a specific amount (weight) of crop material from a grain cart. The operator will typically watch a monitor inside the cab of the tow vehicle that shows how much the crop material, currently in the grain cart, weighs and when the desired weight of crop material has been removed, the operator can stop the crop material from being discharged from the grain cart (typically by shutting gates in the bottom of the grain cart storage hopper or by stopping the auger assembly on the grain cart). However, manually doing this requires a near constant attention by the operator because he or she has to watch the weight of the crop material decreasing on the monitor and manually press a switch or button to stop the crop material from continuing to be discharged out of the grain cart. If this operator is not paying enough attention, he or she could unload too much crop material from the grain cart before he or she realizes it. Alternatively, even if the operator is paying close attention, he or she has to use their best judgement of when to stop the crop material from being discharged. Judging when to stop the crop material from being discharged from the grain cart can be difficult since many grain carts, especially the larger grain carts, can discharge crop material quite fast. If the operator presses the necessary switch or button to stop the crop material too early or too late, the desired weight of crop material will not be discharged out of the grain cart. Additionally, even after the gates are shut by the operator, there is still crop material in the auger assembly, so crop material can continue to be discharged from the grain cart even after the gates have been closed and until all the crop material still present in the auger assembly when the gates were closed have been discharged out of the grain cart.
There have been some attempts to automate the unloading of a predetermined weight of crop material from a grain cart, but they have been overly complex and usually involve modifications or changes to the hydraulics on the grain cart in order to operate.